I built a sweet butane-powered aluminum spud gun for our county's 2nd annual spud gun competition. We took 2nd place in the distance category at 610ft, 1st was at 650ft. We didn't place in accuracy (our friends with the stainless breech loader took that one), so to improve my piece, I'd like to figure out a way to rifle the barrel. Its funny how we got home after the competition and with the first two shots we hit the same spot on a tree 250yds away. Its a 48.5" barrel about 1/4 inch thick. It is welded to the combustion chamber, which is made of a 5.5" ID 1/4" aluminum tube and 1/4" aluminum ends. It's using an $8 stun gun wired to a spark plug for ignition and we feed butane into the chamber via a 100cc syringe. 190cc of butane is about ideal for this cannon. It sits on a tripod because its far easier to aim, I'm too lazy to hold it, and nobody doesn't like a tripod.
Anyway, its a smooth bore, as most spud guns. I know theres a dude who sells rifled PVC barrels, which is really awesome, but PVC won't help an aluminum gun. So, does anyone have any ideas for rifling a 2" aluminum barrel? If it makes any difference, its 5052 aluminum. It doesn't have to be super perfect, but the closer I can get, the better.

Are there any rules that say you have to use a 1x combustion for this competition? If not, even converting that monster to a 2x hybrid would blow the competition out of the water and get rid of the need for a rifled barrel to gain an edge.

Depending on the rules, a larger cannon will allow use of higher sectional density ammo. Launch baker spuds instead of #2 russets.

You seem to have the short fat chamber. Great job on that. You may want to look into a chamber fan and possibly some insulating chamber sleeve inside the chamber to reduce the rate the combustion gas cools.

Are there rules that pertain to the loading process? With the projectile near the muzzle, you could inject a little extra butane (or MAPP) and then push the projectile to its firing position near the chamber. Simple way to make it hybrid I spose.